Integrated neutron-gamma radiation detector with optical waveguide and neutron scintillating material

ABSTRACT

A radiation detector includes a neutron sensing element having a neutron scintillating material at least partially surrounded by an optical waveguide material; and a photosensing element optically coupled to the neutron sensing element. The photons emitted from the neutron sensing element are collected and channeled through the optical waveguide material and into the photosensing element.

This application is a continuation of application Ser. No. 12/027,828,filed Feb. 7, 2008, the entire contents of which are incorporated hereinby reference.

CROSS-NOTING TO RELATED APPLICATION

This application is related to application Ser. No. xx/xxx,xxx, filedMay 17, 2010, entitled “RADIATION DETECTOR WITH OPTICAL WAVEGUIDE ANDNEUTRON SCINTILLATING MATERIAL,” the entire contents of which areincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The teachings herein relate to a detector of ionizing radiation and moreparticularly to a detector for detecting a gamma component and a neutroncomponent.

2. Description of the Related Art

Detection of radioactive materials, particularly those illicitly hiddenin the stream of commerce, requires the availability of a variety ofradiation detection equipment. In particular, Hand-Held RadioIsotopeIdentification Devices (HHRIID) are needed in the field to quicklydetermine the presence of special nuclear material and distinguish itfrom the presence of medical and industrial radioisotopes, as well asfrom normally occurring radioactive material. One possible embodiment ofan HHRIID consists of two optically separated radiation sensors thatemit light and are coupled to a common photodetector. The firstradiation sensor is a neutron sensing component that contains atomicnuclei with a high neutron cross section, such as ⁶Li in a chemicalcompound, such as ⁶LiF, surrounded by particles of a scintillatormaterial, for example, ZnS:Ag, and bound together in an epoxy matrix.The second radiation sensor is a gamma sensing component and consists ofa scintillator crystal with enhanced gamma energy resolution, high gammastopping power, and an atomic composition with very low neutronabsorption cross section. The two radiation sensors are opticallyseparated in such a manner that the light emitted by one sensor does notreach the other sensor in order to avoid optical crosstalk. The HHRIIDmay include a pulse shape discrimination circuit that identifies thesource of light emitted (either by the neutron sensing component or thegamma sensing component based on the difference in scintillation lightdecay times.)

In the detection of neutrons via solid-state scintillation, perhaps themost highly-utilized material stems from a granular mixture of ⁶LiF andZnS:Ag. Each component in this mixture represents “best-of-class”performance (i.e., respectively, neutron capture and luminescence). Forneutron capture, the LiF crystal structure offers one of the highest Liatom densities in solid-state form and maximizes the probability ofneutron interaction, especially if in addition it is enriched in ⁶Li.Furthermore, the absorption of thermal neutrons by ⁶Li induces directdisintegration into alpha and triton particles with no other secondaryradiation. The absence of multiple reaction pathways and/or radiationby-products enables one to optimize the corresponding phosphor to asingle secondary radiation type (i.e., heavy charged particles). Forluminescence, ZnS:Ag is one of the brightest phosphors known and remainsunparalleled in its emission under alpha and triton exposure.

A crucial metric in determining the performance of a neutronscintillator is neutron sensitivity, the number of neutron eventsregistered per incoming neutron flux. This measurement requires thecollection and counting of photons from the neutron scintillator.However, the light output of ⁶LiF/ZnS:Ag materials is limited by twofactors: [1] self-absorption of the emitted light by the ZnS:Agphosphor, and [2] optical attenuation of the emission photons viascattering. The latter arises due to the granular nature of the material(i.e., the multitude of interfaces with index-of-refraction mismatches).The end result is a threshold in thickness beyond which further (useful)light output is unachievable.

Conventional neutron detection approaches typically rely on the opticalcoupling of a thin disk of ⁶LiF/ZnS:Ag composite (<1 mm) to the flat,circular face of a photosensor. For reasons stated above, the neutronsensitivity of this design cannot be improved by increasing thethickness of the disk. Instead, multiple layers of ⁶LiF/ZnS:Ag compositemust be employed, which in turn, create substantial difficulties intransporting the resulting additional light to the photosensor(s).Furthermore, a flat disk may not be the desired shape for neutroncapture. If an application requires the moderation of neutron energies(i.e., reduction to ambient, thermal energies), cylindrical shells arepreferable to disks. For this geometry, the challenge of light transportbecomes even more acute.

In order to improve the total neutron sensitivity of the detector whileproviding a design that reduces both the size and weight of thedetector, an optimal integrated gamma/neutron detector must address theissue of packaging a larger area of neutron sensing composite.

BRIEF SUMMARY OF THE INVENTION

In one aspect, a radiation detector comprises a neutron sensing elementcomprising a neutron scintillating material at least partiallysurrounded by an optical waveguide material; and a photosensing elementoptically coupled to the neutron sensing element, wherein photonsemitted from the neutron sensing element are collected and channeledthrough the optical waveguide material and into the photosensingelement.

In another aspect, a method for detecting radiation comprises:

disposing a neutron scintillating material within an optical waveguidematerial to form a neutron sensing element; and

optically coupling the neutron sensing element to a photosensingelement,

whereby photons emitted from the neutron sensing element are collectedand channeled through the optical waveguide material and into aphotosensing element.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded view of an integrated neutron/gamma detectorincluding a neutron sensing element with an optical waveguide andneutron scintillating material according to an embodiment of theinvention;

FIG. 2 is an exploded view of the neutron sensing element of theneutron/gamma detector of FIG. 1 according to an embodiment of theinvention;

FIG. 3 is a perspective view of the integrated neutron/gamma detector ofFIG. 1 when assembled;

FIG. 4 is an exploded view of an integrated neutron/gamma detectorincluding a neutron sensing element with an optical waveguide andneutron scintillating material according to another embodiment of theinvention;

FIG. 5 is an exploded view of the neutron sensing element of theneutron/gamma detector of FIG. 4 according to another embodiment of theinvention;

FIG. 6 is a perspective view of the integrated neutron/gamma detector ofFIG. 4 when assembled;

FIG. 7 is a cross-sectional view of strands of the neutron sensingelement according to an alternate embodiment of the invention; and

FIG. 8 is a cross-sectional view of strands of the neutron sensingelement according to another alternate embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to FIGS. 1-3, an integrated neutron-gamma radiationdetector is shown generally at 10 according to an embodiment of theinvention. At one end, the detector 10 includes a neutron moderator 12that includes a material that slows down fast neutrons entering themoderator 12, but allowing thermal neutrons and gamma rays to easilypass therethrough. For example, the neutron moderator 12 may includehydrogen, and the like. The moderator 12 includes a cavity 14 that canbe lined with an optical reflector material 16. A gamma sensing element18 is disposed within the cavity 14 of the neutron moderator 12 andsurrounded by another layer of the optical reflector 16 to increase theoptical efficiency of the detector 10. In one embodiment, the gammasensing element 18 comprises a scintillator crystal that emits a lightpulse having a decay time, τ, when a gamma ray collides with the gammasensing element 18. Typical materials for the scintillator crystalinclude, without limitation, crystalline materials with high energyresolution (3% or better at 662 keV) from the Lanthanum halides class(LaBr₃, LaCl₃, LaI₃), as well as solid solutions of these materials.Other dense, bright and fast scintillator materials are useful forincorporation into the gamma sensor 18 as well. For example, thescintillator crystal may be made of a mixed lanthanum halide LaX₃:Ce(X═Cl, Br, I) gamma scintillator material that emits a light pulsehaving a decay time, τ₁, of about 20 nanoseconds. The mixed lanthanumhalides LaX₃:Ce (X═Cl, Br, I) gamma scintillator material hasoutstanding energy resolution that will consequently enablehigh-performance room temperature detectors at considerably lower costwhen compared to current technologies, such as cryogenically cooled highpurity Germanium (HP Ge).

The detector 10 includes a neutron sensing element 20 that is disposedwithin the cavity 14 of the neutron moderator 12. In the illustratedembodiment, the neutron sensing element 20 is disposed proximate thegamma sensing element 18 such that at least a portion of the gammasensing element 18 is disposed within the neutron sensing element 20 fora spatially-compact design, as shown in FIG. 3. The optical reflectormaterial 16 is used in layers that surround both the neutron sensingelement 20 and the gamma sensing element 18 to provide an opticallyefficient design.

The detector 10 also includes a photosensing element or array ofphotosensing elements 24, such as a photodiode or photomultiplier tube(PMT), and the like, which is optically coupled to the gamma sensingelement 18 and the neutron sensing element 20. In the illustratedembodiment, a light collector 22 is disposed between the photosensingelement 24 and the gamma sensing element 18 and the neutron sensingelement 20. The collector 22 is made of an optically transparentmaterial, such as epoxy, plastic, glass, and the like. The purpose ofthe collector 22 is to act as a lens and funnel the photons emitted fromthe gamma sensing element 18 and neutron sensing element 20, which havea relatively large diameter, into the photosensing element 24, whichtypically has a relatively smaller diameter, and increase the opticalefficiency of the detector 10. The collector 22 may be surrounded by anoptical reflector (not shown), if necessary, to further increase theoptical efficiency of the detector 10. It will be appreciated that thecollector 22 can be eliminated when the relative diameters of the gammaand neutron sensing elements 18, 20 are approximately equal to thediameter of the photosensing element 24.

It will be appreciated that the invention can be practiced with anysuitable photosensor acting as the photosensing element, and that theuse herein of the photodiode or PMT as a photosensor is merelyillustrative and non-limiting. The photosensing element 24 outputs asignal, S, indicative by its decay time characteristic of the twodifferent types of photons emitted by the gamma sensing element 18 andthe neutron sensing element 20. Thus, detector 10 needs only a singlephotosensing element 24 to detect the two different types of lightpulses emitted by the gamma and neutron sensing elements 18, 20.

Although in the illustrated embodiment the integrated detector 10includes an array of photodiodes as the photosensing element 24, thedetector 10 may comprise other photosensitive devices. For example,other embodiments of the detector 10 may include a photomultiplier tube,a PIN photodiode, an avalanche photodiode, a Geiger-mode operatingphotodiode, a hybrid photodetector and other similar devices, operatingeither individually or grouped as an array. In short, the photosensingelement 24 is designed to receive and interpret a signal from each ofthe gamma sensing element 18 and the neutron sensor element 20 (each ofthe gamma sensing element 18 and the neutron sensing element 20 being ascintillator and providing and optical output in response to a radiationinteraction). To interpret such signals, the detector 10 may alsoinclude a pulse shaping and processing electronics package (not shown)of a type well-known in the art that processes the signal, S, from thephotosensing element 24 to determine whether a given photon-emittingevent is indicative of radiation interaction in the gamma sensingelement 18 or in the neutron sensor element 20. In the case of gammarays, the electronics also determine the energy of the gamma ray basedon the amount of charge generated in the photosensing element 24 and acalibration procedure with known gamma ray energies from radioisotopicsources. For example, the pulse shape and process electronics packagemay include an analog-to-digital converter (ADC) and also a charge [Q]to digital converter (QDC) (not shown) that receive the signal, S, fromthe photosensing element 24. Each signal, S, is indicative of aradiation interaction in one of the gamma sensing element 18 and theneutron sensing element 20, and has a signal amplitude V₀.

Referring now to FIG. 2, one aspect of the invention is that the neutronsensing element 20 comprises an optical waveguide material 20 asurrounding a neutron scintillating material 20 b, and a centralaperture 20 c of sufficient dimension such that at least a portion ofthe gamma sensing element 18 is capable of being disposed therein. In anembodiment, the optical waveguide material 20 a may be made of, forexample, fused/glassy silica, transparent plastic, and the like. Forexample, the optical waveguide material 20 a may comprise an opticalwaveguide of a type well-known in the art. The neutron scintillatingmaterial 20 b may comprise, for example, a mixture of particles of aneutron capture material, for example, ⁶Li in a chemical compound form,such as ⁶LiF, and a scintillator material, such as ZnS:Ag, both in anoptically transparent epoxy matrix.

The neutron scintillator material has a relatively large cross section(940 barns per Li⁶atom) for thermal neutrons. Upon absorption of athermal neutron, ⁶Li decays into ³H and emits an alpha particle, bothcharged particles with a total kinetic energy of about 4.8 MeV. Thealpha particle and the triton are absorbed by the scintillator material,such as ZnS:Ag, which emits a 450 nm photon having a decay time, τ₂, ofabout 110 nanoseconds, which is different than the decay time, τ₁, ofthe photons emitted from the scintillator crystal of the gamma sensingelement 18. An optimum radius or thickness is utilized to maximize thephoton flux exiting the surface area of the neutron scintillatingmaterial 20 b. For example, the radius of a very thin shell of⁶LiF/ZnS:Ag composite material would be selected to maximize internallight generation, and the thickness of the cylindrical shell would bechosen to optimize emissions from the exterior and interior surfaces.

It will be appreciated that the invention is not limited to using⁶LiF/ZnS:Ag as the neutron scintillating material, and that otherneutron scintillating materials can be disposed within the opticalwaveguide. These materials may include compositions in which both theneutron capture and luminescence functions are embodied within a singlecomposition (i.e., not two as in the case of ⁶LiF and ZnS:Ag). Forexample, liquid or plastic neutron scintillating materials containing⁶Li, ¹⁰B, ¹⁵⁷Gd, and the like, are suitable candidates as neutronscintillating materials.

As described above, interaction of the alpha particle and triton withthe scintillator material, such as ZnS:Ag, provides for photon emissionfrom the neutron scintillating material 20 b. Accordingly, althoughother phenomena may be included or potentially influence signalsgenerated by the LiF/ZnS:Ag component, it should be recognized that theuse of “neutron sensor” accounts for the various aspects and mechanismsthat provide for or are attendant with neutron detection, and thereforethe term “neutron sensor” is not to be limited by the various aspectsand mechanisms.

In the illustrated embodiment, the neutron sensing element 20 comprisesa plurality of concentric cylindrical rings or shells of alternatingoptical waveguide and neutron scintillating material. The rings orshells of material are extended along and are substantially aligned witha longitudinal axis 26 of the detector 10. However, it will beappreciated that the invention is not limited to a particularconfiguration for the neutron sensing element 20 and that many possibleconfigurations for the neutron sensing element 20 are within the scopeof the invention. In one example, the neutron sensing element 20 has aninnermost shell of optical waveguide material 20 a followed byalternating shells of neutron scintillating material 20 b and waveguidematerial 20 a, and an outermost shell of waveguide material 20 a. Inanother example, the neutron sensing element 20 has an innermost solid“core” of either optical waveguide material 20 a or neutronscintillating material 20 b followed by alternating shells of opticalwaveguide material 20 a and waveguide material 20 b, and an outermostshell of waveguide material 20 a (i.e., without the central aperature of20 c); in this latter case, it is not required that the gamma sensingelement 18 be disposed within the central aperture 20 c of the neutronsensing element 20.

It will also be appreciated that the invention is not limited by thenumber and thickness of the alternating shells of waveguide material andneutron scintillating material, and that the number and thickness of theshells can be varied as needed. For example, the neutron sensing element20 may comprise of only three shells of material, e.g., a shell ofneutron scintillating material 20 b disposed between an innermost and anoutermost shell of optical waveguide material 20 a. It will also beappreciated that the invention is not limited by the use of alternatinglayers, and that the invention can be practiced with non-alternatinglayers of optical waveguide material and neutron scintillating material.

Because the optical waveguide 20 a surrounds the neutron scintillationmaterial 20 b, the photon emitted by the neutron sensing material 20 bis collected and channeled to the end of the optical waveguide material20 a. The photosensing element 24, which is optically coupled to theneutron sensing element 20, then detects the photon as a neutron event.To enhance internal reflection of the photon, the optical waveguidematerial 20 a should have (ideally) a refractive index greater than thatof the neutron scintillating material 20 b. If needed, the surfaces ofthe optical waveguide material 20 a may be coated with a thin reflectivelayer (not shown) to induce a “one-way mirror” effect to improveinternal reflection. Ideally, the optical waveguide material 20 a shouldalso have a very high transmission (>90%) at the wavelength of theneutron scintillating material 20 b. It is appreciated that thethickness of the optical waveguide material 20 a and the thickness anddiameter of the core of neutron scintillating material 20 b can beoptimized to maximize the amount of photons collected by the waveguidematerial 20 a and ultimately interpreted as an event.

Referring now to FIGS. 4-6, another configuration of the neutron sensingelement 20 of the detector 10 is shown. In this embodiment, the neutronsensing element 20 comprises a plurality of “strands” of the neutronscintillating material 20 b disposed within the interior of an annularor tubular optical waveguide material 20 a. The “strands” extend alongand are substantially aligned with the longitudinal axis 26 of thedetector 10 to form a multi-layered structure having the centralaperture 20 c such that at least a portion of the gamma sensing element18 can be disposed therein, similar to the cylindrical shell of theneutron sensing element 20 shown in FIGS. 1-3. The “strands” may alsoform the neutron sensing element 20 with a solid core (without thecentral aperture 20 c) in the case where it is not required that thegamma sensing element 18 be disposed within the central aperture 20 c ofthe neutron sensing element 20.

It will be appreciated that the invention is not limited by the numberof layers of “strands” to form the neutron sensing element 20, and thatthe invention can be practiced with an optimal number of layers forphoton collection and transmission to the photosensing element 24.

In the embodiment shown in FIG. 5, each “strand” of the neutron sensingelement 20 comprises a cylindrical tube of optical waveguide material 20a and a solid “core” of neutron scintillating material 20 b disposedwithin the cylindrical tube of optical waveguide material 20 a. However,it should be understood that the invention is not limited to theconfiguration for each “strand” of the neutron sensing element 20 andthat many configurations are within the scope of the invention. Forexample, the “strands” of neutron sensing element 20 can comprise anouter, cylindrical shell of optical waveguide material 20 a and aninnermost concentric solid “core” of optical waveguide material 20 awherein a neutron scintillating material 20 b is disposed between theshell and “core”, as shown in FIG. 7. In an alternate embodiment, the“strands” of neutron sensing element 20 can comprise two concentric,cylindrical shells of optical waveguide material 20 a and a neutronscintillating material 20 b disposed between the two shells, as shown inFIG. 8.

While the invention has been described with reference to an exemplaryembodiment, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the invention. Inaddition, many modifications may be made to adapt a particular situationor material to the teachings of the invention without departing from theessential scope thereof. Therefore, it is intended that the inventionnot be limited to the particular embodiment disclosed as the best modecontemplated for carrying out this invention, but that the inventionwill include all embodiments falling within the scope of the appendedclaims.

1. A radiation detector, comprising: a neutron sensing elementcomprising a neutron scintillating material at least partiallysurrounded by an optical waveguide material; and a photosensing elementoptically coupled to the neutron sensing element, wherein photonsemitted from the neutron sensing element are collected and channeledthrough the optical waveguide material and into the photosensingelement.
 2. The detector according to claim 1, wherein the neutronsensing element comprises a plurality of cylindrical, concentric shells.3. The detector according to claim 2, wherein at least one shell of theplurality of cylindrical shells is substantially aligned along alongitudinal axis of the detector.
 4. The detector according to claim 2,wherein the neutron scintillating material and optical waveguidematerial alternate between each shell of the plurality of cylindricalshells.
 5. The detector according to claim 1, wherein the neutronsensing element comprises a plurality of strands.
 6. The detectoraccording to claim 5, wherein the plurality of strands form a layeredstructure forming at least one cylindrical, concentric shell of theneutron sensing element.
 7. The detector according to claim 5, whereineach strand comprises a solid core of the neutron scintillating materialdisposed within the optical waveguide material.
 8. The detectoraccording to claim 5, wherein each strand comprises a cylindrical layerof neutron scintillating material disposed between a solid core andouter cladding of optical waveguide material.
 9. The detector accordingto claim 5, wherein at least one strand of the plurality of strands issubstantially aligned along a longitudinal axis of the detector.
 10. Thedetector according to claim 1, wherein the optical waveguide materialcomprises an optical waveguide.
 11. A method for detecting radiation,comprising: disposing a neutron scintillating material within an opticalwaveguide material to form a neutron sensing element; and opticallycoupling the neutron sensing element to a photosensing element, wherebyphotons emitted from the neutron sensing element are collected andchanneled through the optical waveguide material and into a photosensingelement.